Human and Nonhuman Primate Lineage-Specific Footprints in the Salivary Proteome

Paleolithic Gene Duplications Primed Adaptive Evolution of Human Amylase Locus Upon Agriculture

Starch digestion is a cornerstone of human nutrition. The amylase genes code for the starch-digesting amylase enzyme. Previous studies suggested that the salivary amylase (AMY1) gene copy number increased in response to agricultural diets. However, the lack of nucleotide resolution of the amylase locus hindered detailed evolutionary analyses. Here, we have resolved this locus at nucleotide resolution in 98 present-day humans and identified 30 distinct haplotypes, revealing that the coding sequences of all amylase gene copies are evolving under negative selection. The phylogenetic reconstruction suggested that haplotypes with three AMY1 gene copies, prevalent across all continents and constituting about 70% of observed haplotypes, originated before the out-of-Africa migrations of ancestral modern humans. Using thousands of unique 25 base pair sequences across the amylase locus, we showed that additional AMY1 gene copies existed in the genomes of four archaic hominin genomes, indicating that the initial duplication of this locus may have occurred as far back 800,000 years ago. We similarly analyzed 73 ancient human genomes dating from 300 - 45,000 years ago and found that the AMY1 copy number variation observed today existed long before the advent of agriculture (~10,000 years ago), predisposing this locus to adaptive increase in the frequency of higher amylase copy number with the spread of agriculture. Mechanistically, the common three-copy haplotypes seeded non-allelic homologous recombination events that appear to be occurring at one of the fastest rates seen for tandem repeats in the human genome. Our study provides a comprehensive population-level understanding of the genomic structure of the amylase locus, identifying the mechanisms and evolutionary history underlying its duplication and copy number variability in relation to the onset of agriculture.

Structurally divergent and recurrently mutated regions of primate genomes

We sequenced and assembled using multiple long-read sequencing technologies the genomes of chimpanzee, bonobo, gorilla, orangutan, gibbon, macaque, owl monkey, and marmoset. We identified 1,338,997 lineage-specific fixed structural variants (SVs) disrupting 1,561 protein-coding genes and 136,932 regulatory elements, including the most complete set of human-specific fixed differences. We estimate that 819.47 Mbp or ∼27% of the genome has been affected by SVs across primate evolution. We identify 1,607 structurally divergent regions wherein recurrent structural variation contributes to creating SV hotspots where genes are recurrently lost (e.g., CARD, C4, and OLAH gene families) and additional lineage-specific genes are generated (e.g., CKAP2, VPS36, ACBD7, and NEK5 paralogs), becoming targets of rapid chromosomal diversification and positive selection (e.g., RGPD gene family). High-fidelity long-read sequencing has made these dynamic regions of the genome accessible for sequence-level analyses within and between primate species.

Emergence of saliva protein genes in the secretory calcium-binding phosphoprotein (SCPP) locus and accelerated evolution in primates

Genes within the secretory calcium-binding phosphoprotein (SCPP) family evolved in conjunction with major evolutionary milestones: the formation of a calcified skeleton in vertebrates, the emergence of tooth enamel in fish, and the introduction of lactation in mammals. The SCPP gene family also contains genes expressed primarily and abundantly in human saliva. Here, we explored the evolution of the saliva-related SCPP genes by harnessing currently available genomic and transcriptomic resources. Our findings provide insights into the expansion and diversification of SCPP genes, notably identifying previously undocumented convergent gene duplications. In primate genomes, we found additional duplication and diversification events that affected genes coding for proteins secreted in saliva. These saliva-related SCPP genes exhibit signatures of positive selection in the primate lineage while the other genes in the same locus remain conserved. We found that regulatory shifts and gene turnover events facilitated the accelerated gain of salivary expression. Collectively, our results position the SCPP gene family as a hotbed of evolutionary innovation, suggesting the potential role of dietary and pathogenic pressures in the adaptive diversification of the saliva composition in primates, including humans.

Gene communities in co-expression networks across different tissues

With the recent availability of tissue-specific gene expression data, e.g., provided by the GTEx Consortium, there is interest in comparing gene co-expression patterns across tissues. One promising approach to this problem is to use a multilayer network analysis framework and perform multilayer community detection. Communities in gene co-expression networks reveal groups of genes similarly expressed across individuals, potentially involved in related biological processes responding to specific environmental stimuli or sharing common regulatory variations. We construct a multilayer network in which each of the four layers is an exocrine gland tissue-specific gene co-expression network. We develop methods for multilayer community detection with correlation matrix input and an appropriate null model. Our correlation matrix input method identifies five groups of genes that are similarly co-expressed in multiple tissues (a community that spans multiple layers, which we call a generalist community) and two groups of genes that are co-expressed in just one tissue (a community that lies primarily within just one layer, which we call a specialist community). We further found gene co-expression communities where the genes physically cluster across the genome significantly more than expected by chance (on chromosomes 1 and 11). This clustering hints at underlying regulatory elements determining similar expression patterns across individuals and cell types. We suggest that KRTAP3-1, KRTAP3-3, and KRTAP3-5 share regulatory elements in skin and pancreas. Furthermore, we find that CELA3A and CELA3B share associated expression quantitative trait loci in the pancreas. The results indicate that our multilayer community detection method for correlation matrix input extracts biologically interesting communities of genes.

Gene communities in co-expression networks across different tissues

With the recent availability of tissue-specific gene expression data, e.g., provided by the GTEx Consortium, there is interest in comparing gene co-expression patterns across tissues. One promising approach to this problem is to use a multilayer network analysis framework and perform multilayer community detection. Communities in gene co-expression networks reveal groups of genes similarly expressed across individuals, potentially involved in related biological processes responding to specific environmental stimuli or sharing common regulatory variations. We construct a multilayer network in which each of the four layers is an exocrine gland tissue-specific gene co-expression network. We develop methods for multilayer community detection with correlation matrix input and an appropriate null model. Our correlation matrix input method identifies five groups of genes that are similarly co-expressed in multiple tissues (a community that spans multiple layers, which we call a generalist community) and two groups of genes that are co-expressed in just one tissue (a community that lies primarily within just one layer, which we call a specialist community). We further found gene co-expression communities where the genes physically cluster across the genome significantly more than expected by chance (on chromosomes 1 and 11). This clustering hints at underlying regulatory elements determining similar expression patterns across individuals and cell types. We suggest that KRTAP3-1, KRTAP3-3, and KRTAP3-5 share regulatory elements in skin and pancreas. Furthermore, we find that CELA3A and CELA3B share associated expression quantitative trait loci in the pancreas. The results indicate that our multilayer community detection method for correlation matrix input extracts biologically interesting communities of genes.

A Catalog of Coding Sequence Variations in Salivary Proteins' Genes Occurring during Recent Human Evolution

Saliva houses over 2000 proteins and peptides with poorly clarified functions, including proline-rich proteins, statherin, P-B peptides, histatins, cystatins, and amylases. Their genes are poorly conserved across related species, reflecting an evolutionary adaptation. We searched the nucleotide substitutions fixed in these salivary proteins' gene loci in modern humans compared with ancient hominins. We mapped 3472 sequence variants/nucleotide substitutions in coding, noncoding, and 5'-3' untranslated regions. Despite most of the detected variations being within noncoding regions, the frequency of coding variations was far higher than the general rate found throughout the genome. Among the various missense substitutions, specific substitutions detected in PRB1 and PRB2 genes were responsible for the introduction/abrogation of consensus sequences recognized by convertase enzymes that cleave the protein precursors. Overall, these changes that occurred during the recent human evolution might have generated novel functional features and/or different expression ratios among the various components of the salivary proteome. This may have influenced the homeostasis of the oral cavity environment, possibly conditioning the eating habits of modern humans. However, fixed nucleotide changes in modern humans represented only 7.3% of all the substitutions reported in this study, and no signs of evolutionary pressure or adaptative introgression from archaic hominins were found on the tested genes.

Slowly Making Sense: A Review of the Two-Step Venom System within Slow (Nycticebus spp.) and Pygmy Lorises (Xanthonycticebus spp.)

Since the early 2000s, studies of the evolution of venom within animals have rapidly expanded, offering new revelations on the origins and development of venom within various species. The venomous mammals represent excellent opportunities to study venom evolution due to the varying functional usages, the unusual distribution of venom across unrelated mammals and the diverse variety of delivery systems. A group of mammals that excellently represents a combination of these traits are the slow (Nycticebus spp.) and pygmy lorises (Xanthonycticebus spp.) of south-east Asia, which possess the only confirmed two-step venom system. These taxa also present one of the most intriguing mixes of toxic symptoms (cytotoxicity and immunotoxicity) and functional usages (intraspecific competition and ectoparasitic defence) seen in extant animals. We still lack many pieces of the puzzle in understanding how this venom system works, why it evolved what is involved in the venom system and what triggers the toxic components to work. Here, we review available data building upon a decade of research on this topic, focusing especially on why and how this venom system may have evolved. We discuss that research now suggests that venom in slow lorises has a sophisticated set of multiple uses in both intraspecific competition and the potential to disrupt the immune system of targets; we suggest that an exudate diet reveals several toxic plants consumed by slow and pygmy lorises that could be sequestered into their venom and which may help heal venomous bite wounds; we provide the most up-to-date visual model of the brachial gland exudate secretion protein (BGEsp); and we discuss research on a complement component 1r (C1R) protein in saliva that may solve the mystery of what activates the toxicity of slow and pygmy loris venom. We conclude that the slow and pygmy lorises possess amongst the most complex venom system in extant animals, and while we have still a lot more to understand about their venom system, we are close to a breakthrough, particularly with current technological advances.

Comparative physiological anthropogeny: exploring molecular underpinnings of distinctly human phenotypes

Anthropogeny is a classic term encompassing transdisciplinary investigations of the origins of the human species. Comparative anthropogeny is a systematic comparison of humans and other living nonhuman hominids (so-called "great apes"), aiming to identify distinctly human features in health and disease, with the overall goal of explaining human origins. We begin with a historical perspective, briefly describing how the field progressed from the earliest evolutionary insights to the current emphasis on in-depth molecular and genomic investigations of "human-specific" biology and an increased appreciation for cultural impacts on human biology. While many such genetic differences between humans and other hominids have been revealed over the last two decades, this information remains insufficient to explain the most distinctive phenotypic traits distinguishing humans from other living hominids. Here we undertake a complementary approach of "comparative physiological anthropogeny," along the lines of the preclinical medical curriculum, i.e., beginning with anatomy and considering each physiological system and in each case considering genetic and molecular components that are relevant. What is ultimately needed is a systematic comparative approach at all levels from molecular to physiological to sociocultural, building networks of related information, drawing inferences, and generating testable hypotheses. The concluding section will touch on distinctive considerations in the study of human evolution, including the importance of gene-culture interactions.

Structurally divergent and recurrently mutated regions of primate genomes

To better understand the pattern of primate genome structural variation, we sequenced and assembled using multiple long-read sequencing technologies the genomes of eight nonhuman primate species, including New World monkeys (owl monkey and marmoset), Old World monkey (macaque), Asian apes (orangutan and gibbon), and African ape lineages (gorilla, bonobo, and chimpanzee). Compared to the human genome, we identified 1,338,997 lineage-specific fixed structural variants (SVs) disrupting 1,561 protein-coding genes and 136,932 regulatory elements, including the most complete set of human-specific fixed differences. Across 50 million years of primate evolution, we estimate that 819.47 Mbp or ~27% of the genome has been affected by SVs based on analysis of these primate lineages. We identify 1,607 structurally divergent regions (SDRs) wherein recurrent structural variation contributes to creating SV hotspots where genes are recurrently lost (CARDs, ABCD7, OLAH) and new lineage-specific genes are generated (e.g., CKAP2, NEK5) and have become targets of rapid chromosomal diversification and positive selection (e.g., RGPDs). High-fidelity long-read sequencing has made these dynamic regions of the genome accessible for sequence-level analyses within and between primate species for the first time.

Traces of dietary patterns in saliva of hominids: Profiling salivary amino acid fingerprints in great apes and humans

Herbivorous animals that regularly consume tannin-rich food are known to secrete certain tannin-binding salivary proteins (TBSPs), especially proline-rich proteins and histidine-rich proteins, as an effective measure to counteract the antinutritive effects of dietary tannins. Due to their high binding capacity, TBSPs complex with tannins in the oral cavity, and thereby protect dietary proteins and digestive enzymes. Although the natural diet of great apes (Hominidae) is biased toward ripe fruits, analyses of food plants revealed that their natural diet contains considerable amounts of tannins, which is raising the question of possible counter-measures to cope with dietary tannins. In our study, we investigated the salivary amino acid profiles of zoo-housed Pan paniscus, Pan troglodytes, Gorilla gorilla, and Pongo abelii, and compared their results with corresponding data from Homo sapiens. Individual saliva samples of 42 apes and 17 humans were collected and quantitated by amino acid analysis, using cation-exchange chromatography with postcolumn derivatization, following acid hydrolysis. We found species-specific differences in the salivary amino acid profiles with average total salivary protein concentration ranging from 308.8 mg/dL in Po. abelii to 1165.6 mg/dL in G. gorilla. Total salivary protein was consistently higher in ape than in human saliva samples (174 mg/dL). All apes had on average also higher relative proline levels than humans did. Histidine levels had the highest concentration in the samples from Po. abelii followed by P. paniscus. In all ape species, the high salivary concentrations of proline and histidine are considered to be indicative of high concentrations of TBSPs in hominids. Given that the species differences in salivary composition obtained in this study correspond with overall patterns of secondary compound content in the diet of wild populations, we assume that salivary composition is resilient to acute and long-lasting changes in diet composition in general and tannin content in particular.

[Developments in Research on Salivary Biomarkers in the Diagnosis of Systemic Diseases]

Saliva, an important biological fluid secreted by oral glands, serves multiple functions. It performs cleaning and protective functions for oral tissues, safeguarding against biological, mechanical and chemical stimuli, while allowing for the sensory perception of taste and temperature. It is also responsible for the preliminary digestion of food. These functions and properties of saliva are attributed to the presence of electrolytes, buffers, proteins, glycoproteins, and lipids in saliva. Recent studies have found that saliva contains biomarkers that are closely connected with the pathophysiological status of the human body, suggesting that saliva makes an ideal biological fluid for drug monitoring and biomarker screening. Therefore, salivary biomarkers can be used as an instrument for physical monitoring and localization of the occurrence of diseases, thereby accomplishing early diagnosis of diseases and assessment of the overall health status of patients. However, the actual application of salivary biomarkers in the diagnosis and treatment of systemic diseases is still not widely available, and the establishment of evaluation criteria and the exploration of its mechanism are not sufficiently investigated. Herein, we reviewed the latest research findings on applying the salivary biomarkers in the diagnosis of systemic diseases.

Influence of tannic acid concentration on the physicochemical characteristics of saliva of spider monkeys (Ateles geoffroyi)

Tannins are a chemical defense mechanism of plants consumed by herbivores. Variations in salivary physicochemical characteristics such as pH, total protein concentration (TP), and presence of proline-rich proteins (PRPs) in animals have been reported as a mechanism to protect the oral cavity when consuming food with variations in pH and tannins. Variations in salivary physiochemistry as adaptations for consuming tannin-rich foods have been found in omnivorous and folivorous primates, but have not yet been reported in frugivorous species such as spider monkeys. We therefore assessed changes in pH using test strips, TP concentration by measuring absorbance at 595 nm in a spectrophotometer and salivary PRPs using the SDS-PAGE electrophoresis technique in the saliva of nine captive spider monkeys in response to the consumption of solutions with different concentrations of tannic acid. The results showed variations in pH, TP concentration and the presence and variation of possible salivary PRPs associated with tannic acid concentration. These findings suggest that spider monkeys may tailor their salivary physicochemical characteristics in response to the ingestion of potentially toxic compounds.

A mechanism of gene evolution generating mucin function

How novel gene functions evolve is a fundamental question in biology. Mucin proteins, a functionally but not evolutionarily defined group of proteins, allow the study of convergent evolution of gene function. By analyzing the genomic variation of mucins across a wide range of mammalian genomes, we propose that exonic repeats and their copy number variation contribute substantially to the de novo evolution of new gene functions. By integrating bioinformatic, phylogenetic, proteomic, and immunohistochemical approaches, we identified 15 undescribed instances of evolutionary convergence, where novel mucins originated by gaining densely O-glycosylated exonic repeat domains. Our results suggest that secreted proteins rich in proline are natural precursors for acquiring mucin function. Our findings have broad implications for understanding the role of exonic repeats in the parallel evolution of new gene functions, especially those involving protein glycosylation.

Saliva and Serum Protein Adsorption on Chemically Modified Silica Surfaces

Biomaterials, once inserted in the oral cavity, become immediately covered by a layer of adsorbed proteins that consists mostly of salivary proteins but also of plasma proteins if the biomaterial is placed close to the gingival margin or if it becomes implanted into tissue and bone. It is often this protein layer, rather than the pristine biomaterial surface, that is subsequently encountered by colonizing bacteria or attaching tissue cells. Thus, to study this important initial protein adsorption from human saliva and serum and how it might be influenced through chemical modification of the biomaterial surface, we have measured the amount of protein adsorbed and analyzed the composition of the adsorbed protein layer using gel electrophoresis and western blotting. Here, we have developed an in vitro model system based on silica surfaces, chemically modified with 7 silane-based self-assembled monolayers that span a broad range of physicochemical properties, from hydrophilic to hydrophobic surfaces (water contact angles from 15° to 115°), low to high surface free energy (12 to 57 mN/m), and negative to positive surface charge (zeta potentials from –120 to +40 mV at physiologic pH). We found that the chemical surface functionalities exerted a substantial effect on the total amounts of proteins adsorbed; however, no linear correlation of the adsorbed amounts with the physicochemical surface parameters was observed. Only the adsorption behavior of a few singular protein components, from which physicochemical data are available, seems to follow physicochemical expectations. Examples are albumin in serum and lysozyme in saliva; in both, adsorption was favored on countercharged surfaces. We conclude from these findings that in complex biofluids such as saliva and serum, adsorption behavior is dominated by the overall protein-binding capacity of the surface rather than by specific physicochemical interactions of single protein entities with the surface.

The Human Salivary Proteome Wiki: A Community-Driven Research Platform

Saliva has become an attractive body fluid for on-site, remote, and real-time monitoring of oral and systemic health. At the same time, the scientific community needs a saliva-centered information platform that keeps pace with the rapid accumulation of new data and knowledge by annotating, refining, and updating the salivary proteome catalog. We developed the Human Salivary Proteome (HSP) Wiki as a public data platform for researching and retrieving custom-curated data and knowledge on the saliva proteome. The HSP Wiki is dynamically compiled and updated based on published saliva proteome studies and up-to-date protein reference records. It integrates a wide range of available information by funneling in data from established external protein, genome, transcriptome, and glycome databases. In addition, the HSP Wiki incorporates data from human disease-related studies. Users can explore the proteome of saliva simply by browsing the database, querying the available data, performing comparisons of data sets, and annotating existing protein entries using a simple, intuitive interface. The annotation process includes both user feedback and curator committee review to ensure the quality and validity of each entry. Here, we present the first overview of features and functions the HSP Wiki offers. As a saliva proteome-centric, publicly accessible database, the HSP Wiki will advance the knowledge of saliva composition and function in health and disease for users across a wide range of disciplines. As a community-based data- and knowledgebase, the HSP Wiki will serve as a worldwide platform to exchange salivary proteome information, inspire novel research ideas, and foster cross-discipline collaborations. The HSP Wiki will pave the way for harnessing the full potential of the salivary proteome for diagnosis, risk prediction, therapy of oral and systemic diseases, and preparedness for emerging infectious diseases.Database URL: https://salivaryproteome.nidcr.nih.gov/.

The evolution and changing ecology of the African hominid oral microbiome

The microbiome plays key roles in human health, but little is known about its evolution. We investigate the evolutionary history of the African hominid oral microbiome by analyzing dental biofilms of humans and Neanderthals spanning the past 100,000 years and comparing them with those of chimpanzees, gorillas, and howler monkeys. We identify 10 core bacterial genera that have been maintained within the human lineage and play key biofilm structural roles. However, many remain understudied and unnamed. We find major taxonomic and functional differences between the oral microbiomes of Homo and chimpanzees but a high degree of similarity between Neanderthals and modern humans, including an apparent Homo-specific acquisition of starch digestion capability in oral streptococci, suggesting microbial coadaptation with host diet.

The oral microbiome plays key roles in human biology, health, and disease, but little is known about the global diversity, variation, or evolution of this microbial community. To better understand the evolution and changing ecology of the human oral microbiome, we analyzed 124 dental biofilm metagenomes from humans, including Neanderthals and Late Pleistocene to present-day modern humans, chimpanzees, and gorillas, as well as New World howler monkeys for comparison. We find that a core microbiome of primarily biofilm structural taxa has been maintained throughout African hominid evolution, and these microbial groups are also shared with howler monkeys, suggesting that they have been important oral members since before the catarrhine–platyrrhine split ca. 40 Mya. However, community structure and individual microbial phylogenies do not closely reflect host relationships, and the dental biofilms of Homo and chimpanzees are distinguished by major taxonomic and functional differences. Reconstructing oral metagenomes from up to 100 thousand years ago, we show that the microbial profiles of both Neanderthals and modern humans are highly similar, sharing functional adaptations in nutrient metabolism. These include an apparent Homo-specific acquisition of salivary amylase-binding capability by oral streptococci, suggesting microbial coadaptation with host diet. We additionally find evidence of shared genetic diversity in the oral bacteria of Neanderthal and Upper Paleolithic modern humans that is not observed in later modern human populations. Differences in the oral microbiomes of African hominids provide insights into human evolution, the ancestral state of the human microbiome, and a temporal framework for understanding microbial health and disease.

Functional Specialization of Human Salivary Glands and Origins of Proteins Intrinsic to Human Saliva

Salivary proteins are essential for maintaining health in the oral cavity and proximal digestive tract, and they serve as potential diagnostic markers for monitoring human health and disease. However, their precise organ origins remain unclear. Through transcriptomic analysis of major adult and fetal salivary glands and integration with the saliva proteome, the blood plasma proteome, and transcriptomes of 28+ organs, we link human saliva proteins to their source, identify salivary-gland-specific genes, and uncover fetal- and adult-specific gene repertoires. Our results also provide insights into the degree of gene retention during gland maturation and suggest that functional diversity among adult gland types is driven by specific dosage combinations of hundreds of transcriptional regulators rather than by a few gland-specific factors. Finally, we demonstrate the heterogeneity of the human acinar cell lineage. Our results pave the way for future investigations into glandular biology and pathology, as well as saliva's use as a diagnostic fluid.

Salivary Fructosamine as a Noninvasive Glycemic Biomarker: A Systematic Review

BACKGROUND

Standard diagnostic and monitoring methods for glycemic status involve invasive sample collection through venous puncture or fingerstick. Recent attention has been focused on exploring noninvasive methods through oral biofluids. Specifically, serum fructosamine has been established as a short-term (2- to 3-wk) marker of disease status in patients with diabetes. Fructosamine measured through noninvasive means such as saliva has shown promise, but its clinical applicability is unknown.

OBJECTIVE

Evaluate the available evidence on using salivary fructosamine as a reliable noninvasive marker to screen and diagnose patients with diabetes mellitus in the clinical setting. A comparative analysis of the correlative accuracy of salivary fructosamine measurements with established blood glycemic biomarkers such as serum fructosamine, blood glucose, and HbA1c will be conducted.

METHODS

Six electronic databases (PubMed, PubMed Central, MEDLINE, EMBASE, Scopus, Cochrane Library) were searched for original research papers (clinical and animal studies) that were relevant to the objective of this systematic review. The search was initiated on May 28, 2020. The systematic review was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Bias risk assessment, overall quality, and level of evidence were based on the Oxford Centre for Evidence-Based Medicine, Appraisal Tool for Cross-Sectional Studies, and Collaborative Approach to Meta-Analysis and Review of Animal Data from Experimental Studies tool. Heterogeneity was assessed using the National Institutes of Health quality assessment tool for cross-sectional studies.

RESULTS

A total of 174 records were identified. Full-text articles screened for eligibility (n = 21) identified only 6 original research articles relevant to the research question and were thus included in the systematic review. The types of studies identified were cross-sectional and in vivo studies. Three studies (3/6) showed positive correlation of salivary fructosamine with blood glucose levels, while 1 study (1/6) demonstrated a positive correlation with glycated hemoglobin (HbA1c). Limitations related to sample size and selection were identified along with a fair level of interstudy heterogeneity.

CONCLUSION

Based on the evidence evaluated, the utility of salivary fructosamine as a noninvasive marker to screen and diagnose patients with diabetes is doubtful. The overall level of evidence was low (IIIB) and the risk of bias was determined to be high.

KNOWLEDGE TRANSFER STATEMENT

Further evidence in the form of large-scale well-controlled studies is needed prior to recommending salivary fructosamine as a noninvasive diagnostic tool for glycemic status in patients with diabetes mellitus.

Salivary proteome of a Neotropical primate: potential roles in host defense and oral food perception

BACKGROUND

Saliva contains a very complex mixture of proteins for defense against microbiological pathogens and for oral food perception. Howler monkeys are Neotropical primates that can consume a mostly leaf diet. They are well known to thrive in highly disturbed habitats where they may cope with a diversity of dietary challenges and infection risks. We aimed to describe the salivary proteome of howlers to contribute to better understanding of their physiology.

METHODS

We analyzed the salivary proteins of wild black howler monkeys (Alouatta pigra), by SDS-PAGE-1-D and Nano LC-MS/MS and categorized them by their function involved in host defense and oral food perception.

RESULTS

Our proteomic analysis identified 156 proteins in howler saliva including a number of host defense peptides that are the first line of defense in mammals, such as defensin, cathelicidin, dermcidin, and lactotransferrin, and proteins with anti-bacterial, anti-fungal, and anti-viral capacity, such as IgA, IgG, IgM, BPI, salivary heat shock 70 kDa protein, beta-2-microbulin, and protein S-100. We also identified key proteins necessary for taste perception, including salivary carbonic anhydrase VI, cystatin D, IgA, and fatty acid-binding protein. Proteins to detect astringent foods were identifying, including four members of cystatins (A, B, C and D), lactoperoxidase, and histidine-rich proteins. No chitinase and amylase were identified as would be expected because howlers do not eat insects and little starch. These findings provide basic information to future studies in oral biology, ingestive physiology, and physiological ecology of mammals and non-human primates.

The parotid secretory protein BPIFA2 is a salivary surfactant that affects lipopolysaccharide action

NEW FINDINGS

What is the central question of this study? The salivary protein BPIFA2 binds lipopolysaccharide, but its physiological function is not known. This study uses a new knockout mouse model to explore the physiological role of BPIFA2 in the oral cavity and systemic physiology. What is the main finding and its importance? BPIFA2 is a crucial surfactant in mouse saliva. In its absence, saliva exhibits the surface tension of water. Depletion of BPIFA2 affects salivary and ingested lipopolysaccharide and leads to systemic sequelae that include increased insulin secretion and metabolomic changes. These results suggest that the lipopolysaccharide-binding activity of BPIFA2 affects the activity of ingested lipopolysaccharide in the intestine and that BPIFA2 depletion causes mild metabolic endotoxaemia.

ABSTRACT

Saliva plays important roles in the mastication, swallowing and digestion of food, speech and lubrication of the oral mucosa, antimicrobial and anti-inflammatory activities, and the control of body temperature in grooming animals. The salivary protein BPIFA [BPI fold containing family A member 2; former names: parotid secretory protein (PSP), SPLUN2 and C20orf70] is related to lipid-binding and lipopolysaccharide (LPS)-binding proteins expressed in the mucosa. Indeed, BPIFA2 binds LPS, but the physiological role of BPIFA2 remains to be determined. To address this question, Bpifa2 knockout (Bpifa2^{tm1(KOMP)Vlcg} ) (KO) mice were phenotyped, with emphasis on the saliva and salivary glands. Stimulated whole saliva collected from KO mice was less able to spread on a hydrophobic surface than wild-type saliva, and the surface tension of KO saliva was close to that of water. These data suggest that BPIFA2 is a salivary surfactant that is mainly responsible for the low surface tension of mouse saliva. The reduced surfactant activity of KO saliva did not affect consumption of dry food or grooming, but saliva from KO mice contained less LPS than wild-type saliva. Indeed, mice lacking BPIFA2 responded to ingested LPS with an increased stool frequency, suggesting that BPIFA2 plays a role in the solubilization and activity of ingested LPS. Consistent with these findings, BPIFA2-depleted mice also showed increased insulin secretion and metabolomic changes that were consistent with a mild endotoxaemia. These results support the distal physiological function of a salivary protein and reinforce the connection between oral biology and systemic disease.